(1) Field of the Invention
The present invention relates to an optical system of a rear projection picture display apparatus for displaying a large picture by projecting the picture displayed on a picture display unit using a projection lens onto a screen.
(2) Description of the Prior Art
A picture display apparatus having high resolution and a large screen has been desirable for commercial uses such as High Vision and industrial uses such as high grade computer displays. With the above applications, projection type picture display apparatus are spreading throughout the market.
Projection type picture display apparatus are classified into two types. One is a two unit type which projects a picture on a screen which is separated from the projector unit. Another is a single unit type in which a projector unit and a screen are built in one enclosure. The latter is called rear projection type.
The former is considerably small, has minimal restrictions in projection distance and is free in projection size. However, if external light comes on the screen, the projected picture is accompanied by black buoying. Therefore, it is desirable to use the former in a dark room (or in a similar type of environment). The latter can display a good picture even under a bright environment because no external light from the projector side comes to the screen. However, the apparatus may require a space corresponding to the projection distance. Thus, the enclosure of the apparatus may be big. This is not desirable for home use.
To make an apparatus small size, two kinds of solutions have been considered in a rear projection type. One solution is to use a projection lens having a short focal distance in order to reduce the projection distance. Another solution is to miniaturize a projection tube, a projection lens, etc. However, after much experimentation in these areas, it is difficult to expect further progress in the future although a small improvements may be made.
One attempt to make the apparatus size small is disclosed in Japanese Patent Laid-Open 2-140792. An optical system for a rear projection picture display apparatus in accordance with the prior art is shown in FIG. 1. The apparatus includes a picture display unit (not shown in the drawing), a beam splitter 2 for partially reflecting and partially transmitting the incident light, a mirror 3 for returning the reflected light from the beam splitter 2 toward the beam splitter 2 again, a louver 4 provided at the rear surface of the beam splitter 2 so that the light in the previously designated range of the incident angle against the perpendicular line to the beam splitter 2 can transmit to the beam splitter 2, and a screen 5.
With the above-mentioned construction, space for the folded light path of the mirror is reduced.
However, because the louver 4 splits transmitting light and not-transmitting light according to its incident angle, the incident angle differs at the center and at the periphery of the screen, and brightness unevenness occurs unless a constant amount of light is transmitted for any incident angle. Further, because the louver 4 has plural light paths, the transmission factor may decrease, and a moire due to a mutual relation with a displayed picture may appear. As a result, picture quality deterioration will be induced.
To solve the above-mentioned problem, a rear projection picture display apparatus includes a picture display unit having a display device for displaying a picture corresponding to an input video signal, a projection lens for projecting and magnifying the displayed picture on the display device, a screen for projecting the picture displayed on the display device through the projection lens and two mirrors on the light path between the projection lens and the screen.
The first plane mirror of the projection lens side is made of a reflecting polarizer and provides a characteristic that transmits light oscillating in the parallel direction to the incidence plane (which is called P-polarized light) among the incident light to the first plane mirror and reflects the light oscillating in the perpendicular direction to the incidence plane (which is called S-polarized light) and fixed at the position to screen the light between a second plane mirror of the screen side and the screen to be displayed with a picture.
The first plane mirror is set so that the incident angle of the light from the projection unit to the first plane mirror is sufficiently larger than the incident angle of the light to the first plane mirror after being reflected at the second plane mirror. A polarizing plate is provided at the rear side of the screen so that the polarized light oscillating in the direction to transmit the first plane mirror is absorbed.
The S-polarized light among the light coming to the first plane mirror from the projection lens is reflected at the first plane mirror. The S-polarized light reflected at the first plane mirror is reflected at the second plane mirror toward the screen keeping its oscillation direction.
The fast plane mirror is located between the second plane mirror and the screen, but because the first plane mirror is made of a reflecting polarizer and the angle relation between the first plane mirror and the second plane mirror is set so that the incident angle of the light traveling to the first plane mirror from the picture display unit is an angle which the first plane mirror can sufficiently display its performance as a polarizer. In addition, the incident angle of the light traveling to the first plane mirror again after being reflected at the second plane mirror is an angle which the fast plane mirror can not sufficiently display its performance as a polarizer. The light reflected at the second plane mirror transmits the first plane mirror and can arrive at the screen, although the first plane mirror locates to screen the light traveling from the second plane mirror to the screen.
Therefore, the space in the enclosure which is necessary to fold the light path by the first plane mirror can be made smaller than the space of a usual rear projection picture display apparatus using surface-coated mirrors or rear surface-coated mirrors as the first and second plane mirrors and the entire apparatus size can be made small.
Picture quality deterioration is protected by providing a polarizing plate between the screen and the first plane mirror to absorb the P-polarized light oscillating in the parallel direction to the incidence plane among the light traveling to the first plane mirror from the projection lens and eliminate harmful light.
Another rear projection picture display apparatus to solve the above-mentioned problem includes a picture display unit having a display device for displaying a picture corresponding to an input video signal, a projection lens for magnifying and projecting the displayed picture on the display device, a screen for projecting the picture displayed on the display device through the projection lens and two mirrors on the light path between the projection lens and the screen.
The first plane mirror at the projection lens side is made of a reflecting polarizer and provides a characteristic that transmits the P-polarized light oscillating in the parallel direction to the incidence plane among the incident light to the first plane mirror and reflects the S-polarized light oscillating in the perpendicular direction to the incidence plane and fixed at the position to screen the light between a second plane mirror at the screen side and the screen to be displayed with a picture.
A quarter-wave plate is attached on the reflecting surface of the second plane minor so that the S-polarized light traveling to the second plane mirror is rotated by 90 degrees at the reflection at the second plane mirror and travels toward the screen as P-polarized light.
The S-polarized light among the light traveling from the projection lens to the first plane minor is reflected at the first plane minor and then, when it is reflected at the second plane mirror the oscillating direction of the S-polarized light is rotated by 90 degrees by passing through the quarter-wave plate on the second plane minor and the S-polarized light is transformed to a P-polarized light and travels toward the screen.
Because the light traveling from the second plane mirror is P-polarized light, it transmits the first plane mirror and can arrive at the screen.
Here, the first plane mirror is made of a reflecting polarizer similar to the first resolving means but no restriction is needed for a relation between the incident angle of the light traveling from the picture display unit to the first plane mirror and the incident angle of the light traveling to the first plane mirror after being reflected at the second plane minor.
According to the second resolving means, because the space in the enclosure can be made small by folding the light path by the first plane mirror, similar to the first resolving means, the entire apparatus size can be made small.
Further, a polarizing plate is positioned between the first plane mirror and the screen to absorb the P-polarized light among the incident light to the first plane mirror. Thus, harmful light can be eliminated and picture quality deterioration is prevented. Providing eliminating means of the light oscillating in a parallel direction to the first plane mirror between the picture display unit and the first plane mirror also increases design freedom.
In the first and second resolving means described above, a picture display device which the oscillating direction of the emitted light is restricted to a specified direction like a liquid christal display panel can be used as a picture display unit and it does not result in a decrease of brightness.